1. Field of the Invention
This invention relates to solar water heating systems, and more particularly to a closed solar heating system wherein an initial charge of water is retained permanently as the heat transport fluid, and which is provided with means to drain a solar collector during periods of cold temperature automatically and with no mechanical moving parts.
2. Description of the Prior Art
A major problem associated with solar heating systems involves the need to protect the solar collectors exposed to the environmental elements from the effects of cold temperatures. In particular, solar collectors are generally susceptible to rupture during periods of extreme cold when a fluid circulating through the collectors is likely to freeze within the collector unless preventive measures are taken.
One preventive measure suggested in the prior art is to add antifreeze to the collector circulating fluid in order to lower the freezing point of this fluid. Anti-freeze, however, is not only expensive, but also can be poisonous and therefore represent a serious potential safety hazard. Exposure to heat can, over a period of time cause the antifreeze to deteriorate and become corrosive, with resultant damage to the piping within the solar system. Furthermore, although an antifreeze mixture does not freeze at lower temperatures, nevertheless the temperature of the mixture does drop to the low ambient temperature. Therefore substantial time is needed to heat the cold antifreeze mixture to a useable temperature as the solar environment of the collector improves.
It has therefore been desirable to protect solar collectors from fluid freezing by draining the collectors during periods of cold temperature. One such system, as disclosed in U.S. Pat. No. 4,044,754 to Cronin et al., employs drainage valves in piping below the level of the solar collector to drain the collector during periods of low temperature. Also provided at the highest point of the system is a vacuum-breaker valve which allows air to enter the water lines to facilitate drainage.
Another solar collector drainage system is disclosed in U.S. Pat. No. 4,027,821 to Hayes et al. wherein a vent line connects the top of the collector to a storage tank and provides an air passageway to the collector from the storage tank to enable drainage of the collector. The vent line is connected to the collector by means of a check-valve and a solenoid actuated valve to initiate drainage of the collector and the vent line. While the systems of the prior art generally provide effective drainage during normal operation, it is seen that they employ mechanically operated valves to initiate drainage. Unfortunately, valves of this type are prone to failure, especially at low temperature when their function is vital to protect the system. In that regard, it is noted that the flow passages of valves tend to be small and are likely to freeze due to condensation or minor back-dripping in the system, and valves are therefore believed to be inherently unreliable drainage components.
It is further noted the "open" drainage systems which utilize valving actuation to admit exterior air into the system during drainage, cannot be easily pressurized due to valve leakage, and therefor are to some extent susceptible to boiling and vaporization at operating temperatures normal to solar heating systems. Since vaporization is most likely to occur at the output of the solar collector, a hydraulic imbalance is created between the supply and return lines to the collector, thereby increasing the load to the system pump, which is also inclined to cavitation under these conditions, further compounding the problems resulting from valve leakage. Furthermore, the "open" system introduces fresh oxygen into the system during each system drainage, which contributes to steady corrosion of the flow passages within the system.
In a prior application, U.S. Application Ser. No. 890,306 the inventor, Embree discloses, as FIG. 1, an arrangement of pipes connected into a sump tank to provide venting of the return pipe line from the solar collector and hence initiating automatic drain-back of the heat transport fluid into the sump tank when the circulating pump is de-energized. That disclosure was an improvement over prior art in that, except for the pump no moving mechanical parts (i.e. valves or flapper) were required in the system, and thus maximum reliability was achieved. In developing that system Embree experimented with and abandoned the use of a check valve as a combination vacuum break vent and bypass flow preventer because under certain conditions the check or flapper valve would fail to open, in effect being stuck in the closed position. The failure of the valve to open appeared to be caused either by water surface tension effects at the flapper valve or by a small particle of dirt or sludge causing the valve to stick. Only one such failure of the valve to open in cold weather could allow the collector and its piping to freeze and rupture; hence Embree's efforts to develop a drain-back system with no moving parts, which were successful and resulted in the reference application Ser. No. 890,306.
That prior disclosure and application Ser. No. 890,306 has two disadvantages in comparison to the present invention; firstly the assembly of pipes connected to and external to the sump tank was awkward and it was not always possible to find space enough to install the assembly, and additionally the pipe connections needed protection from damage in shipment; secondly under certain flow conditions an excess of pump head (otherwise desirable) can cause noisy bypass flow to occur through the "vacuum break section" of that device.